The present invention relates to a method for rendering substrates super hydrophobic.
Plasma-deposited fluorocarbon coatings are often cited in the literature as xe2x80x9cteflon-like coatingsxe2x80x9d because their CFx (0 less than xxe2x89xa62) composition and surface energy can be made very close to that of polytetrafluoroethylene (PTFE,xe2x80x94(CF2xe2x80x94CF2xe2x80x94)n), known on the market as Teflon(copyright).
Plasma coating processes of metals, polymers, and other substrates, with fluorocarbon films are known in the art. As an example, it is known from U.S. Pat. No. 4,869,922 and from other sources, that deposition from continuous (i.e. non modulated) radiofrequency (RF) glow discharges fed with fluorocarbons provides films, layers, tapes, plates, and differently shaped articles made of plastics, metals or other materials, with a thin fluorocarbon coating, with no other material interposed between the coating itself and the substrate. Such coatings are claimed to have very good adherence to the items processed, to be void-free, to be not porous, and to show controlled wettability characteristics, which depend on their the above mentioned patent leads to coatings characterized by static water contact angle (WCA) values lower than 120xc2x0.
Glow discharges treatments are also considered in U.S. Pat. No. 5,462,781 for improving the bondability of an implantable polymer medical device or for changing the wettability of a polymer fabric. Several of the references discussed in this patent confirm non modulated, continuous plasma treatments as a means for varying the inherent WCA of a surface. U.S. Pat. No. 5,034,265 discloses a non modulated, continuous plasma treatment for improving the biocompatibility of vascular grafts with a CFx fluorocarbon coating deposited at the inside wall of the grafts in a proper plasma reactor fed with tetrafluoroethylene (C2F4, TFE) at 0.2 Torr. In the preferred embodiment of the invention no other materials are interposed between the substrate and the coating.
Specifically, the present invention, having the features mentioned in the annexed claims, relates to a modulated plasma deposition process for coating substrates with a thin, well adherent, nonporous, fluorocarbon coating with super hydrophobic properties, i.e. characterized by static water contact angle (WCA) values, measured on a smooth and plane surface, higher than about 120xc2x0, preferably higher than 130xc2x0, more preferably higher than 150xc2x0. Substrates treated with this method have their hydrophobicity markedly improved, e.g. can be made effectively waterproof while maintaining their previous characteristics such as permeability to gases and vapors.
The increased hydrophobicity results also in additional benefits such as prevention of build-up of soiling (e.g. on hard surfaces such as glass, ceramics, metals and other surfaces exposed to dirt), prevention of lumping of powders or granules, aiding in complete emptying of containers which contain hydrophilic materials such as liquid detergent or shampoo bottles or beverage containers or liquid tanks or flowable particle tanks e.g. flour tanks, prevention of contamination and build-up on toothbrushes and bristles. Also by using a metal electrode, made of an antibacterial metal such as silver or gold, in the method according to the present invention an antibacterial property can be provided to the coated surfaces.
The present invention deals with a method of treating polymeric or non polymeric articles for making their surface super hydrophobic, i.e. characterized by static water contact angle (WCA) values higher than about 120xc2x0, preferably higher than 130xc2x0, more preferably higher than 150xc2x0. The method consists of a modulated glow discharge plasma treatment performed with a fluorocarbon gas or vapor compound fed in a properly configured reactor vessel where the substrates are positioned. The plasma process deposits a continuous, fluorocarbon thin film so with super hydrophobic surface characteristics, tightly bound to the substrate.
The substrates of interest for the present invention may include a wide range of materials in form of webs, tapes, films, powders, granules, particles, woven and non-woven layers; substrates can be porous or non-porous, molded or shaped, rigid or flexible, made of polymers, textiles, papers, cellulose derivatives, biodegradable materials, metals, ceramics, semiconductors, and other inorganic or organic materials. Preferably, the substrate is formed into a desired shape or configuration, depending on its intended use, before being subjected to the treatment object of this invention.
When organic synthetic resins are chosen, such substrate materials could be fabricated from polyethylene, polyacrylics, polypropylene, polyvinyl chloride, polyamides, polystyrene, polyurethanes, polyfluorocarbons, polyesters, silicone rubber, hydrocarbon rubbers, polycarbonates and other synthetic polymers.
xe2x80x9cPlasma,xe2x80x9d as used herein, is used in the sense of xe2x80x9clow-temperature plasmaxe2x80x9d or xe2x80x9ccold plasmaxe2x80x9d produced by igniting a glow discharge in a low pressure gas through a power supply. Glow discharges contain a variety of species chemically active and energetic enough to cause chemical reactions with surfaces exposed, i.e. covalent bonding to a suitable substrate material. Cold plasmas, or glow discharges, are generally produced with high frequency (from KHz to MHz and GHz) power supply (HF plasmas). Electrons, positive and negative ions, atoms, excited. molecules, free radicals, and photons of various energies are formed in a cold plasma.
xe2x80x9cModulated plasmaxe2x80x9d means a non continues plasma, HF plasma, i.e. a glow discharge whose driving power is pulsed between a maximum value and zero (ON/OFF pulse) or a fraction of it, at a certain frequency, with a proper pulse generator connected to the main power supply. In the case of ON/OFF pulsed systems, the time ON and time OFF values are among the experimental parameters of the process. Superimposing a triggering ON/OFF pulse to the main high frequency field which generally drives a glow discharge, alternates short continuous discharges with plasma OFF time intervals where active species still exists in the gas phase, but the effects of ions and electrons are strongly reduced. This alternating exposure to two different processes leads to unique surface modifications of substrates, which are very different from those of continuous plasma process, as it will be shown.
xe2x80x9cPlasma depositionxe2x80x9d or xe2x80x9cplasma polymerizationxe2x80x9d is the plasma process that leads to the formation of thin (0.01-2 xcexcm), partly crosslinked, void-free, continuous coatings well adherent to substrates. The molecules of the gas phase are fragmented by energetic electrons, which are able to break chemical bonds; this process leads to radicals and other chemical species which are able to deposit at surfaces inside the vacuum chamber and form a thin, uniform film. The action of the plasma may also affect the surface of a polymer substrate in the early deposition time; energetic species may break bonds in the substrate with possible evolution of gas products, such as hydrogen, and formation of free radical sites which contribute to form covalent bonds between the growing film and the substrate.
It has been found that it is possible to deposit thin fluorocarbon films with super hydrophobic characteristics, i.e. showing a surprisingly high WCA value, even up to about 165xc2x0. The present invention thus provides a modulated plasma process for coating substrates of the type mentioned above, with fluorocarbon films characterized by a WCA value higher than 120xc2x0, preferably higher than 130xc2x0, more preferably higher than 150xc2x0, by using a modulated plasma process, as it will be described.
According to the present invention, fluorocarbon coatings with F/C ratio from about 1.50 to about 2.00 have been deposited, characterized by WCA values higher than about 120xc2x0, such as between about 155xc2x0 and about 165xc2x0. The coatings have been deposited at the surface of different polymer and non polymer substrates such as polyethylene (PE), polypropylene (PP) polyethyleneterephtalate (PET), and paper in form of films and fabrics, glass and silicon, among many. It should be noted that the F/C ratio could be theoretically up to 3, if the coating would be formed only by a mono-molecular layer of CF3 groups. But the formation of intermolecular cross-links and the formation of claims (containing CF2 fragments) which are grafted onto the surface lowers the above theoretical value so that the obtained coatings, notwithstanding the fact that they contain many CF3 groups, have a global F/C ratio in the range of about 1.50 to about 2.00.
The thickness of the coatings depends on the duration of the plasma process at different conditions, and can be kept between 0.01 and 2 xcexcm. It has been found that the nature of the substrate materials does not influence neither the chemical composition nor the thickness of the coatings. Coatings with WCA values up to about 165xc2x0 (e.g. 165xc2x0xc2x15xc2x0) were obtained.
Substrate to be treated are subjected to modulated plasma gas discharge in the presence of at least one fluorocarbon gas or vapor. Specifically, fluorocarbon gases or vapors such as tetrafluoroethylene (TFE,C2F4), hexafluoropropene (HFP,C3F6), perfluoro-(2-trifluoromethyl-)pentene, perfluoro-(2-methylpent-2-ene) or its trimer may be used, TFE being the presently preferred choice. The plasma deposition process is preferably performed by positioning the substrate in a properly arranged plasma reactor, connecting the reactor to a source of a fluorocarbon gas or vapor, regulating flow and pressure of the gas inside the reactor, and sustaining a glow discharge in the reactor with a high frequency electric field in a pulsed (modulated), node by means of a suitable pulsed power supply. The parameters which define the glow discharge treatment includes the feed gas or vapor, its flow rate, its pressure, the position of the substrate inside the reactor, the design of the reactor, the exciting frequency of the power supply, the input power, the time ON and the time OFF of the pulsing system. Substrates, as those listed in the abstract, may be positioned in the xe2x80x9cglowxe2x80x9d region of the discharge, i.e. directly exposed to the plasma, or in the xe2x80x9cafterglowxe2x80x9d region, i.e. downstream respect to the visible glow. The two positions generally result in coatings with different composition and properties; treating the substrates with modulated glow discharge results also in different coatings respect to continuous treatments.